The invention relates to a pressure spring arrangement in accordance with the preamble of patent claim 1.
For parking brakes in commercial vehicles spring-loaded cylinders are usually installed, either alone or in combination with service brake cylinders. To store the required actuating force of the brake and to carry out the working lift, screwxe2x80x94pressure springs are usually used that are pneumatically prestressed.
The design of such pressure or accumulator springs is defined by a plurality of criteria, of which the available installation space and the targeted output force are significant. Maximum output force at a defined working point (for example, xc2xd lift maximum) with simultaneously limited release pressure and limited construction space is possible only with a spring characteristic that is as flat as possible, that is, at a minimum spring rate. It can be influenced, among other things, by the number of windings, since as the number of windings increases, the spring rate decreases. Theoretically the result is an optimally soft spring at a specified maximum tension, when the available construction space for the spring has been completely filled.
Prior art designs make allowance for this fact with diverse measures. Spring sets are used, where two or more springs are connected in parallel. Furthermore, so-called miniblock springs are used, whose windings can immerge into each other at least over some sections. Such types of springs are characterized by the diameter of the winding that decreases in the direction of both ends of the spring (rolled-up spring) and by the cross section of the wire that decreases with the diameter of the winding (DE 35 36 767 A1).
However, the described construction methods for pressure springs of the type under discussion must be regarded as disadvantageous with respect to the production costs of a brake cylinder. Whereas in the first case the use of several springs leads to higher production and assembly costs, in the second embodiment the process of forming the spring rod must be regarded as expensive.
Given this situation, the object of the invention is to provide for brake cylinders of the type under discussion an inexpensive possibility of using as much as possible the available construction space in the spring chamber. In this respect the goal is, as compared to the conventional arrangement, to increase the spring action during a lifting movement under spring load.
The features, disclosed in the characterizing part of patent claim 1, serve to solve this problem.
Due to the fact that the pressure spring is configured as a rolled up spring and that it is braced, on the one hand, against the piston and, on the other hand, against the housing or base of the brake cylinder, the goal is reached that with the specified outer dimensions of the brake cylinder a comparably higher spring force is obtained. That is, with the use of a rolled-up spring as the pressure spring and with the support face, which exhibits a negative roll-off angle and is located at the piston and/or the housing or the base of the brake cylinder the goal is reached that additional work must be generated to deform the ends when tensioning the pressure spring. The consequence. is that the pressure spring absorbs more energy, which during relaxation can be converted into increased spring action.
As a general principle it must be regarded as adequate within the scope of the inventive idea if, when the pressure spring is tensioned, the final winding(s) of the pressure spring exhibit(s) an adequate degree of freedom of the deformation at a negative angle within the respective adjoining spring winding(s). The goal is to feed energy to the pressure spring along its entire spring length during tensioning, thus preferably beginning from both tips of the windings on the two spring ends. In this sense the spring end(s) of the pressure spring could remain within its/their degree of freedom of the deformation without making direct contact with or positioning against the piston or the housing, although the bracing, which takes place for example at 270 degrees, and the fixing in position with respect to the abutment must be regarded as advantageous for guiding and securing the pressure spring so as not to rotate.
Advantageous designs and further developments are disclosed in the other patent claims.